The present invention relates to a golf ball composed of a core, one or more intermediate layer encasing the core, and an outermost cover layer.
Ionic resins, or ionomers, are conventionally used in golf balls. In particular, using an ionic resin having a high acid content is effective when the aim is to increase the rebound or lower the spin rate of the ball. Art that seeks to increase the rebound or lower the spin rate of the ball by highly neutralizing the ionic resin has also been disclosed. For example, methods that involve blending ionomers together, methods in which other thermoplastic resins or additives are blended together with an ionomer, and methods that increase the degree of neutralization of an ionomer itself have been proposed.
Among methods that involve blending ionomers together, a number of techniques which use in combination two ionomers of different weight-average molecular weights have been proposed. For example, U.S. Pat. No. 7,462,113 discloses the use in a cover material of a ternary ionomer having a weight-average molecular weight of from 80,000 to 500,000 in combination with a ternary ionomer having a weight-average molecular weight of from 2,000 to 30,000. Also, U.S. Pat. No. 7,273,903 describes a cover material obtained by blending together a ternary ionomer having a weight-average molecular weight of from 80,000 to 500,000, a binary ionomer having a weight-average molecular weight of from 2,000 to 30,000, and an optional thermoplastic elastomer.
Golf ball materials which use ionomers and blends wherein the content of unsaturated carboxylic acid (sometimes abbreviated below as “acid content”) falls in a specific range have also been disclosed. For example, U.S. Pat. No. 6,267,693 describes an ionomer blend having an acid content of from 18.5 to 21.5%, and U.S. Pat. No. 5,873,796 describes a blend of an ionomer having an acid content of 19% with an ionomer having an acid content of 15%. Also, U.S. Pat. No. 5,222,733 describes a golf ball composition wherein at least 30% of the composition is an ionomer having an acid content of from 20 to 30%.
However, when an ionic resin having a high acid content is highly neutralized, the durability declines. For this reason, such ionic resins are not actively used in golf balls. Also, ionic resins with a high acid content often have too low a melt index (MI), making them difficult to injection mold and sometimes resulting in a poor workability in the manufacture of golf balls.
By improving the materials that form each part of the golf ball in this way, improvements have been made in ball properties such as the rebound and durability. However, there remains room for further improvement. Moreover, when an ionic resin is used in the outer cover layer of a golf ball, this has had the drawback of worsening the scuff resistance of the ball.